In applications where a circuit must be completely on or off with minimal on-state voltage drop, or no danger of injury or damage from leakage current, the electromechanical relay is the only choice. EMRs also are best if heavy surge currents or spike voltages are anticipated or known to occur.

SSRs are indicated where the primary requirement is the ability to perform for tens of millions of operations, or more. Also, they’re often specified where silence is important. If arcing or RFI must be avoided at all costs, SSRs are better.

One biggest disadvantage of using mechanical relay is that it has moving parts with all rising problems. These are:

Rapid switching application or high current loads will shorten contact life.

Generates electromagnetic noise and interference on power lines.

Poor performance when switching high inrush currents.

Normally only one contact available per relay.

Heat Sink required due to voltage drop across switch.

Can switch only AC or DC.

OFF-state leakage current when switch is open.

Normally limited to switching only a narrow frequency range such as 40 Hz to 70 Hz.

Environmental considerations (heat, dust, dirt, water).

Additional details and considerations that can influence your relay choice

Cost – I allude to this in the above bullet points. According to one recent trade article, the average price for SSRs in 2011 was 2.5 times that of EMRs. Plus, if your application is one in which multiple loads need to be switched, the cost disadvantage of the SSR can become much more pronounced.

Size – SSRs are typically smaller than EMRs and that can be a distinct advantage given today’s trend toward ever smaller products. Especially in printed circuit board applications, using an SSR can help conserve valuable space.

Performance in harsh environments – Another point touched on earlier but worth additional commentary. If your relay will be called on to operate in extreme heat (think an Abrams A1 tank, deployed in the Middle East), cold or continuously exposed to inclement weather, the electromechanical relay is, in most cases, your best choice.

Another important consideration is power consumption. According to published reports, the solid state relay consumes 75% less power than the electromechanical relay.

So, in summary, here are my closing thoughts. If your application is going to involve a lot of switching and you need maximum switching speeds, odd are, an SSR will best fit the bill. Switches will occur nearly instantaneously and you can expect a useful relay life of up to 100 million electrical operations. But if your relay will be in a harsh operating environment in an application needing multiple poles and contacts, the old standby EMR is most likely your best bet.

http://arduino-info.wikispaces.com/arduinopower

IMPORTANT NOTE: There is a issue with start-up of Arduino programs that control these relays. All of these 2,4, or 8 relay boards input controls are Active LOW, meaning that setting a pin LOW turns them ON. To assure that no relays activate at Reset or Power-On until you want them to, the initialization sequence in SETUP should be:
digitalWrite(Relay, HIGH);
pinMode(Relay, OUTPUT);